Source: MSDN, search for ApplicationPath samples. Or one is located directly under ApplicationPath docs:
http://msdn.microsoft.com/en-us/library/system.web.httprequest.applicationpath.aspx
with small excuse in Remarks section

Use this property to construct a URL relative to the application root from a page or Web user control that is not in the root directory.

BTW: In "equivalent Servlet API" getContextPath()" they have made decision elminating this sort of concat mistake:

The path starts with a "/" character but does not end with a "/" character. For servlets in the default (root) context, this method returns "".